1. Field of the Invention
The invention relates to a method for enhancing the metallurgical quality of products treated in a furnace, and especially a reheat furnace. This invention applies to any type of product, but more particularly to products treated in a reheat furnace, such as, for example, billets, blooms, slugs or slabs, or any other product used by iron and steel manufacturers in their production line (such as sheet or plate, tube, etc.). The invention relates more particularly to a method of treating a metallurgical product in a furnace, in which the product be treated is introduced into the furnace and then subjected to the desired treatment before being removed from the furnace, the furnace comprising heating means and especially burners for raising the various zones of the furnace to a variable temperature, it being possible for the atmosphere in these various zones to have an identical or different composition depending on the zones in question of said furnace.
2. Related Art
The environment of a steel (or any other product, especially a metal or iron or steel product), when it is raised to a high temperature during a heat treatment, is often an atmosphere which is oxidizing with respect to the metal. This situation may result, on the one hand, in oxidation of the metal with the formation of a surface layer of scale and, on the other hand, in decarburization of the steel with the creation of a carbon concentration gradient near the surface of the workpiece.
The altered region at the surface of these workpieces is essentially composed of two parts (see FIG. 1), one lying on the atmosphere side (upper scale) and the other adjacent the metal (hybrid region).
The upper part generally is composed of three dense oxide layers: a layer of oxide Fe2O3 (hematite), which is very thin (with a thickness of a few microns), a layer of magnetite (Fe3O4) (about 4% of the total scale) and a thick layer of the oxide FeO (wustite) (about 95% of the total scale) which is of greater or lesser porosity depending on the reheat time and the reheat temperature.
The growth of this scale, which follows a parabolic law, is controlled by the diffusion of Fe2+ ions into the wustite and the magnetite and by the diffusion of oxygen O2− into the hematite.
The lower part, or hybrid region, has a greater or lesser thickness depending on the nature of the steel. It is located at the metal/scale interface and consists of a mixture of FeO and products resulting from the reaction of FeO with the oxides of certain alloying elements. This lower part is also composed of a metal region altered by various phenomena, such as decarburization or internal oxidation. Decarburization is a phenomenon involving the solid-state diffusion of carbon, which reacts with the FeO scale (and/or H2O). The permeability of industrial scale to the gaseous products resulting from the oxidation of carbon (especially CO) makes this oxidation at the surface of the metal almost immediate. Decarburization is therefore limited by the diffusion of carbon at the treatment temperature and is favored by the ability of the gases formed (CO) to escape from the scale-steel interface.
Depending on the thermal profile imposed and on the composition of the atmosphere (especially the O2, H2O and CO2 contents), the iron or steel products may be oxidized (scale) and decarburized (this being the more so in the case of high-carbon steels). In both cases, the steel manufacturer will have to subject his workpieces to an additional operation aimed at eliminating these surface defects. Whereas the oxide layer may be removed by various descaling techniques, the decarburized layer, that forms an integral part of the workpiece, cannot be easily “erased”: the surface of the product is devoid of some of its carbon atoms, thereby degrading the mechanical properties on the surface of the product (longevity, hardness, etc.).
The oxidation or decarburization of steel in a reheat furnace thus results in a loss of raw material, which is called “loss on ignition”, and a degradation of the surface properties of products, which are prejudicial to the steelmaker.
A major constraint, which will also affect the final quality of the product at the end of the reheat process, is the final temperature of the product and its thermal homogeneity, this being so whatever the heating history that has taken place in the furnace (time spent at certain temperature levels, slower production rate following a rolling mill incident, etc.). Any lack of thermal inhomogeneity will cause structural defects and a posteriori mechanical embrittlement of the finished products. These defects may also cause certain parts of the rolling mill (especially rolling-mill stands) to be stopped or even broken.
Any optimization of the metallurgical quality of the product must meet this constraint with regard to the thermal homogeneity of the product. During operation of the furnace by the operator, control of and compliance with the temperature rise of the product are key factors in ensuring in the end that the thermal homogeneity constraint is met.
A person skilled in the art knows that, to avoid decarburization and oxidation, it is recommended to work in a protected atmosphere by substoichiometric combustion (using a fuel-rich mixture generating a neutral or even reducing atmosphere with respect to steel). This method is employed in galvanizing processes (see, for example, Galvanisation et aluminiage en continu [Continuous galvanizing and aluminizing] by E. Buscarlet, Technique de l'ingénieur [Engineering Techniques], 1996.
It is also known, from U.S. Pat. No. 4,415,415, to treat products in an atmosphere containing at least 3% oxygen by volume, and to do so over the entire length of the furnace, thereby inevitably resulting in the formation of scale but making it possible to control the quality of the scale, which, under these conditions, becomes non-adherent and easily removable.
Patent EP-A-0 767 353 also proposes to vary the furnace atmosphere by zoning the furnace, that is to say by isolating the furnace into several chambers within which a highly oxidizing atmosphere is recommended, so as to be able to control the formation and quality of the scale. In this case, the loss on ignition is not reduced, but on the contrary is increased, only the quality of the scale being controlled.
The various methods known from the prior art therefore suggest that the products either be treated in an oxidizing atmosphere or in a reducing atmosphere.
The use of these various methods also has an additional drawback in the case of the treatment of steel products. This is because it is important to be able to measure the oxidizing or reducing character of the atmospheres involved. The only information available during implementation of these processes is provided by measurement probes located either in the roof, that is to say far from the surface of the products, or in the flue of the furnace. These measurements are therefore not representative of the composition of the atmosphere which interacts directly with the product. In general, the only measurable parameter of the atmosphere is the oxygen content. This information is generally insufficient—because the fact that the amount of oxygen in the smoke leaving the furnace is zero does not necessarily mean that the furnace atmosphere in contact with the metal workpieces is reducing with respect to steel (see, for example, Combustion Engineering and Gas Utilisation, published by British Gas, 1992, page 23). According to the Applicant, the species H2O and CO2 also have an oxidizing role on the charge and are involved in scale formation reactions and in decarburization mechanisms. At the present time, it is not known how to measure these species simply and quickly.
To operate the furnace and meet the final constraint of thermal homogeneity of the product, the operator adopts an initial temperature profile of a given product for a given furnace, depending on the type of charge and of production. This profile is either known to the operator, because of his know-how, or is calculated from charts, or else calculated using suitable software.
The only information available for the operator and/or the furnace operation software are the measurements delivered by one or more thermocouples located in the roof of the furnace. These thermocouples are placed far from the charge and are not representative of the heat flux received by the charge beneath the burners. It is therefore necessary to estimate the relationship which links the roof temperature (which is measured) and the temperature of the charge (useful information). This relationship is either empirical (based on the operator's know-how) or calculated using furnace operation software.
Not only is this measurement only an indirect measurement of the necessary information, but the estimated relationship may prove to be less and less accurate upon aging of the furnace, of the thermal characteristics of the various charges and variations in the type of fuel used.
Finally, this measurement is a measurement at a certain point, usually located on the axis of the furnace and it does not take into account possible variations in said parameter over the entire width of the furnace.
The fact of not having measurements made as close as possible to the product has the consequence that the characteristic times of the process for heating these products is not known exactly. Yet it has been found that these characteristics have a major influence on the oxidation and decarburization kinetics of said products, it being possible that an incorrect estimation of these times has serious consequences as regards the final metallurgical quality of the product.